A beam forming network combines multiple signals to produce a composite signal that points in a particular direction in space. The beam forming network can be used for transmit, receive, or transmit and receive applications and is often capable of generating multiple simultaneous beams each of which benefit from the full radiating aperture. Various types of beam forming networks that form fixed spatial beams are known, including a Butler matrix and a Blass matrix.
A typical Butler matrix is made of a variety of quadrature hybrids, fixed phased shifters, and extensive coaxial cables to connect the components. The typical Butler matrix forms a group of multiple beams fixed in space, where the beams are non-steerable and each beam points to a unique position in space. Depending on whether transmitting or receiving, a Butler matrix connects each radio frequency (RF) input to the matrix to a unique beam output, or connects each RF output to the matrix to a unique beam input. Typically, the Butler matrix is physically large, expensive, only operates over a narrow bandwidth of usually about 10%, and suffers from high RF loss, often greater than 20 dB.
Additionally, a typical Blass matrix is made of a variety of fixed phased shifters or delay lines, directional couplers, and transmission lines. The typical Blass matrix provides for an arbitrary number of beams to be transmitted or received to or from an arbitrary number of radiating elements. As with the Butler matrix, a conventional Blass matrix forms a group of multiple beams fixed in space, where the beams are non-steerable and each beam points to a unique position in space. One typical embodiment involves each radiating element feeding a single RF signal path (also referred to as a row). The beams are formed by each column sampling energy from each row. A wideband beam forming/sensing network is formed by inserting a fixed time delay along each row between the beam coupling points.
A Blass matrix uses a variety of directional couplers that are situated along the rows whose coupling coefficients ascend in value as one gets farther from the radiating element. The coupling values ascend in value since each instance that energy is coupled off the incoming signal, a smaller outgoing signal results. By having coupling values ascend, all samples from the various rows couple equal signal levels into each beam.
Various disadvantages of a typical Blass matrix includes that it may be very large, heavy, and expensive, especially when implemented in a waveguide with delay lines. Moreover, even if such a Blass matrix is implemented in planar media like a microstrip or stripline, the implementation is still physically large and expensive. Furthermore, a typical Blass matrix is very lossy, even if couplers with small coupling values are used and only generates non-steerable beams.
As stated above, a prior art beam forming network matrix may include a digital fixed phase shifter and phase generating quadrature hybrid. In the prior art, a typical implementation for a digital phase shifter uses a switched delay line architecture resulting in a solution that is physically large and operates over a narrow band of frequencies due to its distributed nature. Another typical digital phase shifter implements a switched high-pass low-pass filter architecture, which has better operating bandwidth compared to a switched delay line but is still physically large. Also, the phase shifter is often made on gallium arsenide (GaAs). Though other materials may be used, GaAs is a higher quality material designed and controlled to provide good performance of electronic devices. However, in addition to being a higher quality material than other possible materials, GaAs is also more expensive and more difficult to manufacture. The typical phased array components take up a lot of area on the GaAs, resulting in higher costs. Furthermore, a standard phase shifter has high RF power loss, which is typically about n+1 dB of loss, where n is the number of phase bits in the phase shifter. Another prior art embodiment uses RF MEMS switches and has lower power loss but still consumes similar space and is generally incompatible with monolithic solutions.
Furthermore, the typical components in a phased array antenna are distributed components that are therefore frequency sensitive and designed for specific frequency bands.
Quadrature hybrids or other differential phase generating hybrids are used in a variety of RF applications. In an exemplary embodiment, quadrature hybrids are for power combining or power splitting. In an exemplary embodiment, the outputs of a quadrature hybrid have substantially equal amplitude and approximately a 90° phase difference. In another typical embodiment, the quadrature hybrid is implemented as a distributed structure, such as a Lange coupler, a branchline coupler, or a ring hybrid. Other quadrature hybrids, such as a magic tee or a ring hybrid, result in a 180° phase shift. In general, a quadrature hybrid is limited in frequency band and requires significant physical space. Moreover, the quadrature hybrid is typically made of GaAs and has associated RF power loss on the order of 3-4 dB per hybrid when used as a power splitter and an associated RF power loss of about 1 dB when used as a power combiner.
Hybrids may be configured as in-phase power combiners and in-phase power splitters, which are also used in a variety of RF applications. In an exemplary embodiment, the outputs of an in-phase hybrid have approximately equal amplitude and approximately zero differential phase difference. In another exemplary embodiment, the inputs of an in-phase hybrid configured as a power combiner encounter substantially zero differential phase and amplitude shift. In a typical embodiment, the in-phase hybrid is implemented as a distributed structure such as a Wilkinson hybrid. In general, an in-phase hybrid is limited in frequency band and requires significant physical space. The in-phase hybrid is typically made of GaAs. Moreover, the in-phase hybrid generally has associated RF power loss on the order of 3-4 dB per hybrid when used as a power splitter and an associated RF power loss of about 1 dB when used as a power combiner.
Thus, a need exists for a beam forming network that is not frequency limited. Furthermore, the beam forming network should be able to be manufactured on a variety of materials and with little or no associated RF power loss. Also, a need exists for a beam forming network that uses less space than a similar capability prior art network, and is suitable for a monolithic implementation.